The present invention relates to alkoxysilane-functionalized and allophanate-functionalized urethanes, methods for the preparation thereof, coating compositions comprising these and to the use thereof.
Polyurethanes have been established for many decades as high-value components for paint, adhesive, sealant and plastics systems. It is possible here for additional alkoxysilane groups to play an important role, for example with regard to network density, chemical resistance and scratch resistance, primarily through the formation of siloxane and polysiloxane structures.
Molecules not only having alkoxysilane groups but also comprising isocyanate groups offer the option of introducing the functionalities that result as reaction products, siloxanes and polyurethane groups, through one component. Such substances have long been in use, for example in the form of isocyanatoalkyltrialkoxysilanes.
Alkoxysilane-terminated polyurethanes prepared from isocyanatoalkyltrialkoxysilanes and alcohols are also known and are used, for example, for producing highly crosslinked, rigid coating compositions (e.g. WO 2013/189882 A2). However, if these alkoxysilane-terminated polyurethanes are used as sole binder in systems that cure at room temperature, coatings are obtained with only moderate hardness.
Allophanate-containing binders have long been known. Alkoxysilane-functionalized allophanates are also known. There are several types that can be distinguished here, which are shown below, but correspond neither in terms of structure nor in the application to the alkoxysilane-functionalized allophanates according to the invention.
For instance, the allophanates III (1) described in WO 2008/043722 A1 are obtained by reacting NCO-terminated allophanate-containing polyurethanes I (1) with alkoxysilanes II (1) reactive to isocyanate (e.g. aminoalkyltrialkoxysilane). The allophanate groups here are therefore in the centre of the polyurethane chain and the alkoxysilane function is attached via the terminal isocyanate group in the context of a urea function (structure III (1), equation 1).

DE102005041953 A1 describes the reaction of a polyol I (2) having a mean molecular weight of 3000-20 000 g/mol with an excess of isocyanatopropyltrimethoxysilane II (2) so as to result, after polyurethane formation III (2), in the formation of an allophanate IV (2) having two alkoxysilane functions per allophanate unit.

In DE102005041954 A1, a polyurethane I (3) is admixed with isocyanatopropyltrimethoxysilane II (3) and the mixture is heated until allophanate structures are formed. In this case, the alkoxysilane group is attached to the terminal nitrogen of the allophanate group (III) (3) (equation 3).

J. Kozakiewicz et al. published, in Progress in Organic Coatings 72 (2011) 120-130, the reaction of isocyanatopropyltrimethoxysilane I (4) with methanol to form the corresponding urethane II (4) and subsequently with hexamethylene diisocyanate trimer III (4). In the highly viscous allophanate IV (4) resulting therefrom, the alkoxysilane function is appended on the tertiary central amine of the allophanate group (equation 4).

In the use described, the allophanate function serves as blocking agent for the hexamethylene diisocyanate trimer which was used as crosslinker for hydroxy-functionalized polyester polyols.
Even now, there exists a need for novel, silane-containing binders which have specific properties.